KINETICS OF UPTAKE AND INCORPORATION OF VALINE IN SEA URCHIN EMBRYO

Chemical and kinetic studies of uptake and incorporation of 14C-valine have been carried out on developmental stages of the sea urchin, Lytechinus. Valine is taken up by active transport as indicated by a concentrative influx of 14C-valine, saturation kinetics of the Michaelis-Menton type, and a high Q10. Back-exchange occurs during early development up to the mid-blastula stage. Radiochromatographs of acid-soluble extracts show 82-90 per cent of the label in valine and the remaining in 4-5 other low molecular weight compounds. Hydrolyzates of acid-insoluble fractions contain 85-90 per cent of the label in valine. A slow loss of label in carbon dioxide indicates an oxidation of pool valine. At all developmental stages the time course of uptake of 14C-valine is exponential; whereas the time course of incorporation appears to be linear with no appreciable initial lag phase. Uptake and incorporation rates exhibit a nonlinear relationship for different concentrations of exogenous valine, with uptake being more responsive to changing concentration than incorporation rate. Exogenous 12C-valine, to a degree dependent on the concentration, suppresses incorporation of endogenous pool 14C-valine. The kinetic data are interpreted as follows. The amino acid pool is not an obligatory intermediate for incorporation of exogenous valine. At concentrations of exogenous valine which incompletely saturate the transport system, valine for incorporation is drawn partially from the pool. With increasing concentration of exogenous valine less valine is drawn from the pool as the incorporating system shifts to using exogenous valine. At sufficiently high concentrations, when the transport system is saturated, pool valine is used very little or not at all and exogenous valine is incorporated directly. Incorporation rates of exogenous valine thus may give absolute values for rates of protein synthesis. Uptake and incorporation rates were measured from early cleavage through gastrulation using concentrations of exogenous valine giving maximal suppression of incorporation of pool valine. © 1968.